Lamellar ceramic body



United States Patent [72] Inventor Marshall. Norwalk 1 [56] Referen es Cited Watkins Glen, New York UNITED STATES PATENTS 21f; $32 1964 3,083,123 3/1963 Narias 106/62 E plnemed 1970 3,184,531 5/1965 McCreightetaL. 106/62 I 9 [73] Assignee Coming classworks I 3,239,322 3/1966 Carter 65/33 Coming, New York 3,285,761 11/1966 Hare et a1, 106/62 a corporation of New York [54] LAMELLAR CERAMIC BODY 19 Claims, 1 Drawing Fig. 1

[52] US. Cl 126/144, 161/165 [51] lnt. Cl F23m 5/00,

F27d H00 Field of Search 33, 43;263/36,48,46;'757 9Z23/252A; 123/191A; 126/144T; 106 46, 62; 1l7/70A, B, 125, 123, 169; 181/61, 62, 63; 161/182, 213, 165; 138/140-143; 106/58; 13/35; 266/43 Primary Examiner-Robert F. Burnett Assistant Examinen-Linda M. Carlin AttorneySughrue, Rothwell, Mion, Zinn and Macpeak and magnesia, wherein each adjacent layer is a material having a different coefficient of thermal expansion.

Patented Sept. 15, 1970 INVENTOR MARSHA-LL H. NORWALK AAZL OMM W ATTORNEYS LAMELLAR CERAMIC BODY The present invention relates to a ceramic body. More particularly, this invention relates to a laminated ceramic body comprising a plurality of layers of ceramic materials.

With the present interest in space flight and the recent commercial development of gas turbine engines, a tremendous need has been created for improved combustion chamber liners. The gas turbine engine requires a combustion chamber liner which can operate at temperatures as high as 1800C for extended periods of time. The chamber liner must also be highly resistant to thermal shock, since it will be subjected to extreme and almost instantaneous temperature changes, for example, from room temperature to 1300C. A further requirement is that the liner have a thermal expansion coefficient compatible with some readily available metal which will allow it to be used in contact with a metal. In order to render the combustion chamber liner commercially feasible, it must be capable of economical production and not be subject to fatigue after thousands of thermal cycles.

According to the prior art, one approach to providing combustion chamber liners comprises the development of a monolithic ceramic body which would have high thermal shock resistance and which would also be capable of operating at elevated temperatures. However, no known materials capable of operating at temperatures up to about l800C are re sistant to thermal shock. None of the low expansion therinal shock resistant materials can operate at these extreme temperatures.

A second approach to this problem was the development of metal ceramic laminates. These metal ceramic laminates obtain thermal protection from a series of mechanically bonded ceramic and metal layers in which the sublimation or oxidation of the metal lowers the temperature of the base piece. Laminates of this construction have very short useful lives and are not suitable for operations requiring thermal protection for extended periods.

Accordingly, it is an object of this invention to provide a high-strength thermal shock resistant ceramic body capable of operating at temperatures up to about 1800C.

More particularly, it is an object of this invention to provide a strong laminar ceramic combustion chamber liner comprised of a plurality of alternate layers of precracked ceramic material, wherein no two adjacent layers are of the same material and the layers are of a thickness such that injurious thermal stresses are not generated by sudden changes in temperatures.

in the drawing, the single figure is a vertical cross-section of a combustion chamber liner of this invention.

These and other object of this invention are accomplished by providing a laminated structure comprised of alternate layers of ceramic materials, no two adjacent layers being of the same material. Each individual layer is extremely thin, on the order of between 0.005 inch to 0.015 inch, or at least less than about 0.025 inch. It is preferred to employ layers having a thickness of from between 0.005 inch to 0.010 inch, with layers 0.008 inch thick being a preferred average. By using ceramic layers within these thickness ranges, injurious thermal stresses will not be generated within the materials by sudden changes in temperature.

A wide variety of ceramic compositions may be used in forming the laminates of this invention. Generally, any two ceramic materials having approximately the same firing shrinkage, but substantially different coefficients of thermal expansion, and being non-reactive toward each other at maximum use temperature can be used. It has been found that ceramics n an expan i qqsf am f zsrit a .usu ly on the order of 25 40 X exhibit extremely good thermal shock resistance when used in these laminates. In a preferred embodiment, the laminate is formed from alternating layers of fused spinel and fused magnesia. As used in this specification and pv s h s t e erm i sli s wnlym magnesium aluminate, i.e., MgO Alzod- Spinel has an expansion coefficient of about 88 X 10- and magnesia an expansion coefficient of about 120 to 140 X 10*.

Laminates can also be formed from alternating layers of the following pairs of ceramic materials:

a. Fused spinel and a mixture of fused magnesia and fused spinel containing at least about 40 percent by weight of fused magnesia, and

b. Fused magnesia and a mixture of fused magnesia and fused spinel containing at least about 40 percent by weight of fused spinel.

When mixtures of fused spinel and fused magnesia are used,

it is preferred to employ equal parts by weight mixtures.

Since the compositions calculated in weight percent do not differ substantially from those calculated in volume percent, this specification and the appended claims will refer to the mixtures of fused magnesia and fused spinel in terms of weight percent. it will be understood by those skilled in the art that similar results can be obtained by preparing these composi tions in volume percent without departing from the scope of the invention.

Of course, it is not intended that the scope of the present invention be limited by the specific materials disclosed since many other ceramic materials having the properties discussed above can also be employed.

In one embodiment of this invention, the top and bottom layers of the laminate are formed from the ceramic material having the lower coefficient of thermal expansion. Thus, when the laminate is formed from alternating layers of fused spinel and fused magnesia, the top and bottom layers of the laminate are fused spinel. On cooling, compression layers, caused by the greater contraction of the bonded fused magnesia layers, are formed.

Generally, the laminated ceramic combustion chamber liners of this invention are formed by first preparing suitable slip compositions and then forming one ceramic layer at a time at room temperature. After a ceramic layer is formed, it is dried and then the next layer formed and dried. After a suitable number of layers have been formed, the composite is fired.

Slips of the materials to be used are prepared by techniques well known in the art. When a hygroscopic ceramic material, for example, fused magnesia, is employed, a non-aqueous slip should be used. An example of such a slip is a solution of benzene with l5 weight percent of oleic acid as a detlocculant. Suitable slip compositions usually contain 75 percent by weight of solids.

The ceramic layers are then formed by either dipping a mold into the slips or by washing the slips over the surface of a mold. The length of time which the mold remains in the slip and the amount of slip used for washing are dependent on the thickness of the layer desired. When a 75 percent by weight fused spinel or fused magnesia slip is employed, layers having a thickness of between 0.005 inch and 0.015 inch can be formed by immersing the mold in the slip composition for 10 to 60 seconds. Layers of this thickness can also be formed by washing the mold with the slip for about the same period of time.

In forming these laminates, two slip compositions are used and the layers of ceramic are built up alternately until the desired total thickness of the liner is reached. A short drying period between the formation of adjacent layers can be used. This can be readily done by placing the coated mold in a warm, for example, about F, air stream for approximately one to two minutes.

After a suitable number of alternate layers have been formed, the piece is then dried. The drying temperature is not critical and can be varied within usual drying temperature ranges. For example, the pieces may be dried at a temperature of between 25C and 200C with drying at about 45C overnight being preferred.

Following the drying, the green piece is then fired. The fiting temperatures commonly employed for the particular ceramic composition can be used at this stage. For laminated bodies of combinations of fused magnesia and fused spinel having total thicknesses of between 0.125 inch and 0.250 inch, temperatures of between l500C and 1650C for a period of from 1 to 1% hours are suitable.

The ceramic combustion chamber liners of this invention may be formed in plaster, refractory, or paper molds. If a plaster mold is used, the pieces are removed from the mold after the drying period. If a refractory mold has been used, the pieces do not have to be removed from the mold, but may be fired in the mold. Similarly, a paper mold does not have to be removed from the green piece, since it will burn off during the firing cycle. With these considerations in mind, it is preferable to form the ceramic laminate in a refractory or paper mold, since they provide support for thin, fragile, green pieces.

In forming these laminates, the individual layers can be separated during casting by washing a thin burn-out layer over each refractory layer after it is cast. By using a separating layer which is completely oxidized during the firing step, there will be sufficient bonding between the adjacent ceramic layers to develop a unitary structure upon firing. A suspension of percent by weight corn starch in benzene has proven suitable for this purpose.

The combustion chamber liners of this invention can be used for a wide variety of insulating purposes. Generally, they can be used wherever an insulating material highly resistant to thermal shock and having an operating temperature up to 1800C is needed. Many uses for these combustion chamber liners in space vehicles and gas turbine engines will readily suggest themselves to those skilled in the art. The external dimensions of the liner are chosen so that it will snugly fit the interior surface of the combustion chamber. Generally, the liner will be between 0.125 inch and 0.250 inch in thickness. The number of layers per inch of liner thickness will depend upon the individual layer thickness, which is generally found to be best kept in the range of 0.005 inch to 0.015 inch, and preferably between 0.005 inch and 0.010 inch (e.g., 0.008 inch).

With reference to the single figure, there is shown a vertical cross-sectional view of a combustion chamber liner of this invention. The liner 10 is in the form of a hollow frustrum comprised of alternating layers 12 and 14 of fused spinel and fused magnesia respectively. This particular liner, which has a thickness of 0.125 inch, contains 12 alternating layers of spinel and magnesia, each layer being approximately 0.01 inch in thickness. The liner has a top inside diameter of 3.125 inches and a top outside diameter of 3.250 inches. The bottom inside diameter is 4.375 inches with the bottom outside diameter being 4.500 inches. The height of the liner is approximately six inches.

The following examples will serve to illustrate certain preferred embodiments of this invention.

EXAMPLE I A combustion chamber lining containing alternating layers of fused and fused magnesia is formed by first preparing two non-aqueous slips. A first slip is prepared by adding 72 pounds, 14 ounces of fused spinel having a particle size of less than 10 mesh to a solution containing 9,318 cc of benzene and 455 cc of oleic acid. The composition is then placed in a 25 gallon ball mill using 237 pounds of alumina balls and is ground for 16 hours. A second slip composition is formed by adding 80 pounds, five ounces of fused magnesia having a particle size of less than 100 mesh to a solution containing 1 1,851 cc of benzene and 547 cc of a preformed solution of 33 percent oleic acid in benzene. The resulting mixture is placed in a 25 gallon ball mill employing 237 pounds of alumina balls and ground for 11 hours.

A plaster mold having a cavity in the form of a hollow frustum six inches in height with a top diameter of 3.25 inches and a bottom diameter of 4.5 inches is immersed in the fused spinel slip for 10 seconds. The mold is then removed from the slip and the spinel layer dried by directing a room temperature air stream into the interior of the mold for about two minutes. Following this, the mold is dipped into a suspension containing 10 percent by weight of corn starch in benzene. A thin corn starch containing layer is thereby formed on the surface of the spinel layer.

Following this, the mold is dipped in the fused magnesia slip and allowed to remain in the slip for 10 seconds. The fused magnesia layer is then dried by blowing a F air stream into the interior of the mold cavity for about one minute. The mold is then immersed in the corn starch-benzene suspension.

Alternate layers of fused spinel and fused magnesia are formed one upon another by the above technique until a ceramic body having a thickness of 0.125 inch is formed. The body contains seven layers of fused spinel and six layers of fused magnesia, with the two exterior surfaces of the body being formed of fused spinel.

The plaster mold containing the ceramic laminate is then placed in an oven at a temperature of 45C and dried for 18 hours. Following this, the plaster mold is removed and the fragile but self-supporting laminate is placed in an oven and fired at a temperature of 1550C for one hour.

The ceramic laminate is then cooled to room temperature. The thermal shock properties of this laminate are tested by placing the laminate, which is at room temperature, in the exhaust gas stream of a gas fired furnace. This gas stream has a temperature of about 1300C. It is found that the laminate prepared in accordance with example 1 did not fail even after repeated cycles.

A monolithic part of the same configuration as the laminate of example 1 is prepared in a manner identical to that of exampie 1, except that only the fused magnesia slip is employed. The monolithic ceramic part failed on the first exposure to the hot exhaust gases. Thus, the laminated ceramic combustion chamber liners exhibit thermal shock resistance properties far in excess of those of the monolithic parts.

EXAMPLE 2 In a manner similar to that employed in example 1, a laminated ceramic combustion chamber liner comprised of alternating layers of fused spinel and an equal part by weight mixture of fused magnesia and fused spinel can be formed. The fused spinel containing slip is prepared as in example 1. A second slip is prepared by mixing equal volumes of the fused spinel slip and the magnesia slip which were prepared as described in example 1.

Alternate layers of fused spinel and the mixture of fused magnesia and fused spinel are formed in a manner identical with that employed in example 1. The finished liner had a thickness of 0.250 inch and was formed by firing the green preform at a temperature of 1,5 50C for one hour.

The final product withstands repeated thermal shocks from a temperature of room temperature to 1300C.

EXAMPLE 3 A laminated ceramic combustion chamber liner comprised of alternating layers of fused magnesia and an equal part by weight mixture of fused magnesia and fused spinel can be formed by the procedure of example 1. The fused magnesia slip is prepared as in example 1. A second slip is prepared by mixing equal volumes of the fused magnesia slip and the fused spinel slip which were prepared as described in example 1.

After forming alternating layers of fused magnesia and the mixture of fused magnesia and fused spinel by the technique of example 1, the part is dried at 45C for 18 hours, the mold removed, and the green part fired at 1550C for one hour.

The finished liner, having a total thickness of 0.250 inch, is strong and exhibits good thermal shock resistance.

The improved thermal shock resistance of these laminar ceramics can be largely attributed to the precracked state of the ceramic layers. Adjacent layers of the structure are intentionally composed of ceramic materials which differ considerably in thermal expansion coefficient. Consequently, large macrostresses are set up in the layers during cooling from the firing temperature because of the incompatibilities of the length changes in the adjacent layers. These stresses cause extensive localized cracking between and within the layers. An additional type of precracking occurs in laminates of the type described in example 2, where a mixture of two dissimilar crystalline phases, say, magnesia and spinel, is used in alternate layers. Because of the large difference in thermal expansion coefficient between these phases, microstresses arise in the grains within an individual layer during cooling from the firing temperature. When these micros tresses exceed the strengths of the individual grains or of the bonds between the grains, microcracking occurs across or between the grains, respectively. One effect of the macroand microcracks is to impart a flexibility to the laminate which permits thermal stresses to be relaxed during temperature changes. Another beneficial effect is that these small cracks tend to inhibit the propagation of large cracks which would ordinarily cause failure of the article.

While there have been shown and described and pointedout the fundamental novel features of the invention as applied to the preferred embodiment, it will be understood that various omissions and substitutions and changes in the form and details of the laminates illustrated may be made by those skilled in the art without departing from the spirit of the invention. It is the intention, therefore, to be limited only as indicated by the scope of the following claims.

I claim:

1. A high-strength thermal shock resistant precracked laminar ceramic body comprising a plurality of alternate layers of a first ceramic material selected from the group consisting of spinel and magnesia, and a second ceramic material selected from the group consisting of magnesia and an equal part by weight mixture of magnesia and spinel, wherein each layer is of a thickness of between 0.005 inch and 0.015 inch and no two adjacent layers are of the same material.

2. The article of claim 1 wherein each layer is of a thickness of between 0.005 inch and 0.010 inch.

3. The article of claim 1 wherein said body is a ceramic combustion chamber liner in the form of a hollow frustum.

4. A high-strength thermal shock resistant precracked laminar ceramic body comprising a plurality of alternate layers of magnesia and spinel, each layer being of a thickness of between 0.005 inch and 0.015 inch.

5. The article of claim 4 wherein each layer is of a thickness of between 0.005 inch and 0.0l0 inch.

6. The article of claim 4 wherein said body is a ceramic combustion chamber liner in the form of a hollow frustum.

7. The article of claim 4 wherein the exterior layers of said ceramic body are spinel.

8. A high-strength thermal shock resistant precracked laminar ceramic body comprising a plurality of alternate layers of spinel and a mixture of magnesia and spinel containing at least about 40 percent by weight of magnesia, each layer being of a thickness of between 0.005 inch and 0.015 inch.

9. The article of claim 8 wherein each layer is of a thickness of between 0.005 inch and 0.010 inch.

10. The article of claim 8 wherein said body is a ceramic combustion chamber liner in the form of a hollow frustum.

11. The ceramic body of claim 8 wherein the alternate layers are of spinel and an equal part by weight mixture of spinel and magnesia.

12. A high-strength thermal shock resistant precracked laminar ceramic body comprising a plurality of alternate layers of magnesia and a mixture of magnesia and spinel containing at least about 40 percent by weight of spinel, each layer being of a thickness between 0.005 inch and 0.0l5 inch.

13. The article of claim 12 wherein each layer is of a thickness of between 0.005 inch and 0.010 inch.

14. The article of claim 12 wherein said body is a ceramic combustion chamber liner in the form of a hollow frustum.

15. The ceramic body of claim 12 wherein the alternate layers are of magnesia and an equal part by weight mixture of spinel and magnesia.

16. A high-strength, thermal shock resistant laminar ceramic body which is prepared by the process of:

forming a plurality of distinct layers having a thickness of 0.005 to 0.025 inch of a first ceramic material selected from the group consisting of magnesia and magnesia aluminate spinel; forming a plurality of distinct. layers having a thickness of 0.005 to 0.025 inch of a second ceramic material selected from a group consisting of magnesia and an equal part by weight of a mixture of magnesia and magnesia aluminate spinel, wherein said ceramic materials are selected such that the coefficient of expansion of the first ceramic material is different from that of the second ceramic material; applying-a burn out layer to each of said ceramic layers; alternately contacting the distinct layers of first ceramic material with the distinct layers of second ceramic material so that each layer is separated from each adjacent layer by said burn-out layer, heating the assembled body to a temperature sufficient to oxidize said burn-out layer and thereby bond each of said adjacent layers; and

cooling the assembled body so as to provide a unitary precracked structure.

17. The article of claim 16 wherein said body is a ceramic combustion chamber liner in the form of a hollow frustum.

18. The article of claim 16 wherein said first ceramic material is magnesia and said second ceramic material is spinel and wherein the thickness of each layer is between 0.005 inch and 0.0l5 inch.

19. The article of claim 16 wherein said first ceramic material is spinel and said second ceramic material is a mixture of magnesia and spinel containing at least about 40 percent by weight of magnesia. 

